A very significant limitation on the operation of an integrated circuit device is the efficient extraction of heat. Unless the circuit device is provided with an efficient heat transfer mechanism to maintain it within a predetermined operating temperature range, the speed and power of the device are severely limited. Excessive overheating of the integrated circuit device may cause its destruction.
The problem of heat removal from an integrated circuit device is increased when the device is mounted on a supporting substrate with solder terminals for connecting the device to electrically conductive traces on the substrate. In such a solder bonded device the major portion of the heat is usually removed from the back side of the device, through the substrate, to a heat exchanger in thermal contact with the substrate. The introduction of very large scale integration circuit devices, and the dense placement of a plurality of such devices on a single multi-layer, or "thick film," ceramic substrate has significantly increased the heat flux to be removed.
In typical prior art cooling systems, a heatsink has been placed in thermal contact with the surface of the substrate having no circuit devices. A gaseous or liquid coolant circulating over or through the heatsink has been used to dissipate heat from the heatsink. To prevent malfunction of the devices, it is critically necessary that there be intimate contact between the substrate and the heatsink, without clearances therebetween.
However, natural warpage in the substrate cause air pockets at the substrate-to-heatsink interface which interfere with the efficient flow of heat from the substrate to the heatsink. In prior art cooling systems the air pockets have been reduced by tightly coupling the substrate to the heatsink by adhesives or mechanical connecting means. However, any mismatch between the thermal expansion characteristics of the substrate and that of the heatsink will stress an adamantine connection. Shear stresses, which occur during the natural heating cycles of the devices, will eventually cause fatigue in the substrate which can ultimately lead to fractures of the brittle substrate, and failure of the devices mounted thereon.
In addition, it is desirable to allow for the repair of the devices mounted on the substrate. Therefore, the heatsink should be easily removable. The traditional solutions include the use of adhesives, gaskets, and thermal greases at the substrate-to-heatsink interface.
The main disadvantage of solutions which comprise adhesives is that even for flexible adhesives a relatively strong bond is formed between the substrate and heatsink. The rigidity of the adhesive bond increases the risk of cyclical stress related fatigue failures. Furthermore, although adhesives comply well with the surface irregularities of the substrate, they are generally poor thermal conductors and contribute to the overall thermal resistance at the substrate-to-heatsink interface. In addition, adhesion bonding techniques require expensive dispensing equipment, cumbersome curing chambers, and do not easily permit the removal of the heatsink to repair the devices on the substrate.
Solutions which include gasket type of materials are easier to install than adhesives, and generally produce stress free mating at the substrate-to-heatsink interface. However, gasket type materials almost totally lack compliance necessary to compensate for the unevenness of the substrate, and also, the thermal conductivity of gaskets is relatively poor.
Greases are similar in thermal performance to adhesives, that is relatively poor, but greases have the advantage that they produce stress free mating at the substrate-to-heatsink interface. Also greases conform well to surface irregularities in the substrate, thus minimizing the formation of air pockets at the interface. However, greases tend to migrate or flow away from the interface, decreasing, over time, their effectiveness as a mating material, and like adhesives, greases require dispensing equipment.
Accordingly, the known solutions for thermally mating a substrate to a heatsink increase the cost of assembly, and moreover, do not always guarantee a stress free, thermally efficient interface, decreasing the overall performance and reliability of the integrated circuit devices.
Therefore, it is desirable to provide an apparatus and method for reliably mating a substrate to a heatsink which allows for the removal of the heatsink for repair of the circuit devices, which is simple to assemble, and which uses readily available inexpensive materials that have good thermal conductive characteristics.